Occultations
An occultation can be described as occurring when a body passes in front of and obscures a more distant body.
Lunar and asteroidal occultations are the main types under study by this section of the AAQ. The occulting bodies are the Moon and various asteroids, and the objects being occulted are background stars.
Lunar Occultations
As the Moon traverses the sky between new and full Moon, the unilluminated portion leads and so the stars can easily be monitored until occulted by the approaching lunar limb. After full Moon only the brightest stars are visible before occultation because of the bright limb, but re-appearances from behind the dark trailing limb are readily seen. At times the ragged southern or northern limbs will ‘graze’ stars which blink on and off behind lunar mountains. Travel to an observing site along the ‘graze path’ could be considered for favourable grazes. |
Lunar Occultation
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Asteroidal Occultations
Whilst the predictions for lunar occultations are certain and accurate to within several seconds, it is much more difficult to predict an occultation for a rocky body a hundred kilometres in size and perhaps five hundred million kilometres away. Though astrometry has vastly improved the accuracy of these predictions, now in the order of several asteroid track widths, it is still somewhat ‘hit and miss’, and unless an occultation occurs, the time and angle of the merging and separation of the images are noted to refine the time and actual path of the track. |
Asteroidal Occultation
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Planetary Occultations
There are the rarer but very impressive occultations of stars by planets, such as the event in 1971 when Jupiter occulted the bright double star Beta Scorpii. |
Jovian Occultation
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Equipment
To observe and properly record these events, you will need:
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a telescope and a method of ascertaining time - eg short wave radio, telephone time signal,
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a method of keeping time - eg stopwatch, tape recorder application.
Electronic means, for example low light security CCD camera and VCR, 'CCD drift method' are becoming popular.
Purpose And Results
These observations are not made merely for the purpose of intellectual curiosity. Rather they are timed and reported to improve and refine our understanding of the orbits of the various bodies. For asteroidal occultations and lunar grazes, resolution of the profile of the asteroid and lunar limb can be obtained to amazing accuracy, limited effectively by the spacing of the observing stations.
The more common lunar occultations, in particular, can provide unusual events such as stepped occultations and short fades. These can be accounted for by the star being double; very poor ‘seeing’; or a sympathetic slope, particularly for events near the north and south limbs. Occasionally however, previously unrecorded double stars are discovered by this means.
For more information, including details on how to undertake an occultation observation and predictions of upcoming occultations contact the Occultations Section Director.
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Astronomical Association of Queensland 2006. www.aaq.org.au
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